Milliamps and death

[windmill john]

Catchy title huh!

Okay, I’m old enough not to be embarrassed by this question. Let’s not get off topic and talk about AC and its issues, I’m sticking with DC for this question.

I need some clarity… as a beginner… If 100 milliamps can kill, how come in some videos you can see the host twisting wires together in a live circuit at 100 milliamps with no issue?
Now the voltage was 12 volts. I just need clarity. The circuit was complete, no faults. At what point does the danger become an issue? Does it depend on voltage? I’m aware that’s the force etc.
Can this be answered simply without anecdotal stories? Thanks.

[Siwastaja]

With 12V, 100mA was not flowing. If somebody claimed it, they measured wrong. I = U/R. Human body has some certain R.

[Ian.M]

... or more correctly, has some variable R, depending on contact area, skin thickness, condition and dampness, or a much lower R if a conductor penetrates through the skin or the skin is significantly damaged.   Your personal R isn't reliably high enough to protect against shock, so don't count on it to save you when working on circuits carrying more than a few tens of volts.

[bostonman]

Low DC voltage can't break skin resistance. Although, it's probably safe to practice never touching live wires.

DC is dangerous because, unlike AC, it never crosses zero. Every 8.3ms AC crosses zero, so you theoretically have a moment to pull away. DC remains constant (i.e. direct current), so you can become fused to it.

Correct me if I'm wrong, but wet hands touching the positive of a car battery terminal (while some other part of the body is touching battery ground through the metal of the car - or directly to the negative battery terminal) can be deadly because the liquid would reduce the resistance, correct? For this reason, it's why you can lay your finger across a 9-volt battery without getting a shock, but touch the terminals to your tongue and you'll get a jolt.

[TimFox]

As a generalization, a circuit through the human body will see relatively high resistance through the skin (which depends on how wet it is) and relatively low resistance through the interior (wet tissues with lots of salt).

[BeBuLamar]

Catchy title huh!

Okay, I’m old enough not to be embarrassed by this question. Let’s not get off topic and talk about AC and its issues, I’m sticking with DC for this question.

I need some clarity… as a beginner… If 100 milliamps can kill, how come in some videos you can see the host twisting wires together in a live circuit at 100 milliamps with no issue?
Now the voltage was 12 volts. I just need clarity. The circuit was complete, no faults. At what point does the danger become an issue? Does it depend on voltage? I’m aware that’s the force etc.
Can this be answered simply without anecdotal stories? Thanks.

There is no 100mA going thru the host body. In fact he wouldn't be able to hold it if 5mA is going thru his body. So when you watch the video what make you think there is a 100mA or more going thru the host body? I have work on some static eliminator and other high voltage system (20,000V and more) but with the current limit to 5mA. I do not know how much current would go thru a person body when he/she touches it but it would be less than 5mA. It doesn't actually kill anyone with the shock but the reaction of the person getting the shock have been known to cause serious accident.

[Terry Bites]

The typical resistivity of dry skin is about 25,000Ω cm, too high to allow much current to flow. Dry skin has modest insulating properties but this will break down easily when enough voltage is applied. 30V is now considered the upper bound for safe working. (eg a truck battery). Put your fingers across the phone line for a big 48V suprise. Dont.

Even microamps can kill under adverse conditions. A current entering via a skin lesion or puncture wound can affect the nervous system. Put your fingers across a 9V battery. Nothing. Then put it on your tongue. Crikey!  Wanna try that with 24V or maybe 48V Now imagine what a few volts in your precious wet innards. Nerves get twitchy at about 60mV.

Even a very small DC current will cause electrolytic burns and necrosis.
A small, usually pulsed DC current sometimes used to move charged drug molecules under the skin. (Iontophoresis)

[Siwastaja]

... or more correctly, has some variable R, depending on contact area, skin thickness, condition and dampness, or a much lower R if a conductor penetrates through the skin or the skin is significantly damaged.   Your personal R isn't reliably high enough to protect against shock, so don't count on it to save you when working on circuits carrying more than a few tens of volts.

And many, many other things, as equivalent circuit not being anywhere near a resistance, but we need to start from somewhere and get to the simplest possible model that explains away all the "it's amps that kill and volts that jolt" type absolute bullshit.

And constant resistance model is pretty good first order approximation for that. There is voltage, and current relative to it results. Period.

[IanB]

I need some clarity… as a beginner… If 100 milliamps can kill, how come in some videos you can see the host twisting wires together in a live circuit at 100 milliamps with no issue?
Now the voltage was 12 volts. I just need clarity. The circuit was complete, no faults. At what point does the danger become an issue? Does it depend on voltage? I’m aware that’s the force etc.
Can this be answered simply without anecdotal stories? Thanks.

Yes, it can be answered very simply. The measure that counts is the number of milliamps going through your heart, and also for how long. A hundred milliamps flowing in a wire won't mean a thing. A hundred milliamps flowing through your fingers might hurt, but it won't kill you.

What causes current to flow though your chest? A combination of high enough voltage and low enough skin resistance, with a circuit that goes from hand to hand, or hand to feet. Skin resistance matters, so that wet skin is much worse than dry skin. That's why electricity and bathrooms don't mix.

What is a high enough voltage? Roughly speaking, more than 50 V, but there is no specific value. On the other hand, 12 V is not enough in normal circumstances.

[iMo]

The damage is a combination of the current and voltage and the resistivity of your body parts exposed and the path through your body.
A couple of uAmps through your heart may stop it beating (ie. the pace maker regulates the heart rhythm where the currents are almost nil).
The power_loss=voltage*current creates a heat, which may evaporate your fingers or your leg in a fraction of a second. Also the current can make chemical changes in the body liquids/electrolytes (DC especially).
So - it depends. 100mAps through your body would require the voltage and the resistivity such the 100mA could flow. The U*I creates heat which could smoke your tissues. Your nerves are sensitive to the currents as well (your nerves conduct electrical impulses from/to your brain), the path of the current colliding with the important nerves could immobilize you, or stop your breathing, heart, etc.
Thus you cannot say "this current or voltage will do this or that" to your organism, without knowing above details.

[bdunham7]

At what point does the danger become an issue? Does it depend on voltage? I’m aware that’s the force etc.
Can this be answered simply without anecdotal stories? Thanks.

The simple answer is that lower voltages can't cause enough current to flow through your body to cause harm.  The current in the conductors is irrelevant.

The complex answer is that there is quite a bit of variance in results depending on the situation and thus there is not a well defined voltage cut-off line below which you are safe.  The human body isn't really a resistor--maybe it is more like an electrolytic capacitor.  There are any number of standards related to the acceptable voltages in direct external contact with people.  Internal contact is a different matter and swallowing even a 3V coin cell can cause serious medical issues. 

https://en.wikipedia.org/wiki/Extra-low_voltage

[windmill john]

Thanks all, much appreciated.

It all looked safe, but that was visual, I just needed the technical/theoretical answer.

To answer one point above, the circuit was tested and showed 100 milliamps. The poster then undid the connection by hand, hence my question.

If you are happy to accept silly questions (not silly to me ) it’s appreciated. I’m covering all safety aspects before I delve deeper into the subject.

[nightfire]

In appliance testing for work safety, the "Berührungsstrom (german) " (touch current) is measured via a 2k Resistor against earth to simulate the human body.
In reality, depending on some conditions like hand to hand or feet, or adult or child, it obviously varies a bit, but for own rough "over the thumb" calculations something like 1k Ohm is a pragmatical value to use.

And with skin, the voltage needed to push the first current through is a bit higher than what is needed to sustain the current flow.

Doing some math: 12V=1000 Ohms * x Amps
=>0,012 A

With 2000 Ohms obviously half, so 6mA.

And  not to forget: If someone touches a live circuit, we talk about a voltage divider the human body presents, so not all current will flow through the human body, becaus according to Kirchhoffs Law it will divide proportionally to the conductivity of the current paths.

[Siwastaja]

To answer one point above, the circuit was tested and showed 100 milliamps. The poster then undid the connection by hand, hence my question.

You wrote "twisting wires together", so it seems 100mA was flowing through the wires which were twisted together, never through the human being.

[jonpaul]

5 ma is sufficient to cause ear disruption.

Resistance of body depends on probe area, skin penetration, humidity, etc.

Pins inseteed just below the skin surface can get substantial current flow with just 10..30V

SELV = Safety Extra Low Voltage is defined as 42 V DC or peak.

Huge amount of shock/hear/medical /safety info/regs on this topic.

100 Ma is lethals 100%

Jon

[BeBuLamar]

Thanks all, much appreciated.

It all looked safe, but that was visual, I just needed the technical/theoretical answer.

To answer one point above, the circuit was tested and showed 100 milliamps. The poster then undid the connection by hand, hence my question.

If you are happy to accept silly questions (not silly to me ) it’s appreciated. I’m covering all safety aspects before I delve deeper into the subject.

The current if flowing thru the circuit whatever it is but not the host body. I have a heater that draw 500A at 5V and people touching the bare cable without getting shock. Because the 500A is going thru the heater and not the body. There is some current going from the 5V bare wire thru the human body and to the ground but it's very very small and he doesn't get shock.

[bostonman]

Also, a good practice is to keep your left hand in your pocket when dealing with high voltage.

Easier said than done, but, if you accidentally touch a life wire, it will hopefully go down the right side of your body thus bypassing your heart.

[IanB]

There is some current going from the 5V bare wire thru the human body and to the ground but it's very very small and he doesn't get shock.

Why do you say that? Most low voltage supplies are isolated, so there is no path to ground for any current to flow.

[Zero999]

Low DC voltage can't break skin resistance. Although, it's probably safe to practice never touching live wires.

DC is dangerous because, unlike AC, it never crosses zero. Every 8.3ms AC crosses zero, so you theoretically have a moment to pull away. DC remains constant (i.e. direct current), so you can become fused to it.

Correct me if I'm wrong, but wet hands touching the positive of a car battery terminal (while some other part of the body is touching battery ground through the metal of the car - or directly to the negative battery terminal) can be deadly because the liquid would reduce the resistance, correct? For this reason, it's why you can lay your finger across a 9-volt battery without getting a shock, but touch the terminals to your tongue and you'll get a jolt.

You're wrong on both counts.

DC is actually less dangerous than mains frequency AC, for the same current. AC doesn't pass through zero for anywhere near long enough to mitigate any shock. It has a higher peak voltage, compared to DC. 240V is 325V peak, whist 240VDC, never gets above 240V. 50 or 60Hz also disrupts the rhythm of the heart, better than steady DC.

Even if the skin is very wet, it's very difficult to difficult to get a lethal shock from 12VDC. It might be possible if a large area of skin is broken, but still highly unlikely.

[Traceless]

There is an "urban legend" that someone was killed by their handheld DMM which was powered by a 9V battery (I'll leave it to the professionals here to confirm or debunk that). According to this source even a current as small as 0.01mA can cause ventricular fibrillation and thus potentially kill a human under certain circumstances. On the other end of the spectrum most of us have already received static shocks for instance after walking on a carpet and touching some metal object afterwards. Apparently the voltage built up prior to those shocks can be easily several kV.

[TimFox]

Electroplating commonly uses direct current, and there is a safety hazard due to liquids, DC, and other chemical problems (especially with cyanide solutions).

[jpanhalt]

Also, a good practice is to keep your left hand in your pocket when dealing with high voltage.
Easier said than done, but, if you accidentally touch a life wire, it will hopefully go down the right side of your body thus bypassing your heart.

That "one-hand" theory is often cited and unfortunately is interpreted wrong.  I researched its history once.  The real reason for recommending "one hand" is to lessen the chance of touching a live wire  (e.g., two hands have twice the probability). 

Of course, one is often wearing shoes  with low conductivity.  The presumed path is probably wrong.  Remember, the heart is not directly between the two arms, it is located quite low in the chest in what is called the mediastinum.  If the geometric path makes a difference, arm to contralateral/opposite leg should be most dangerous.  There is some evidence for that in humans, but the strongest evidence is from  early experiments in dogs.  See cited references here: https://www.researchgate.net/publication/242705238_A_review_of_hazards_associated_with_exposure_to_low_voltages

Geddes et al. (1973) found the current needed for fibrillation using the upper extremity-to-upper extremity path is about three times greater than for when current is applied between the fore and hind limbs.

It is often stated that DC is more dangerous than AC, based on the theory that one can release from AC easier at the zero crossing point, but the evidence from actual electrocution is very weak. Here is what the previously cited review states (page3):

AC (e.g. 60 Hz sinusoidal) is considered more likely to induce hazardous electric shocks than dc current (Camps et al. 1976; Reilly 1998). Dalziel and Lee (1969) found 10-400 Hz currents most effective in inducing involuntary hand muscle contraction. DiMaio and DiMaio (2001) considered 39-150 Hz the most lethal.  Kugelberg (1976) found frequencies between 12-60 Hz most effective in inducing fibrillation of the human heart.

This review gives more insight into the reason AC is more dangerous: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2763825/

Alternating and direct current
Membranes of excitable tissues (eg, nerve and muscle cells) will pass current into cells most effectively
when an applied voltage is changing. The skin is somewhat similar in that it passes more current when
the voltage is changing. Therefore, with alternating current, there is a continuous changing of the
voltage, with 60 cycles of voltage change occurring per second. With alternating current, if the current
level is high enough, there will be a feeling of electric shock as long as contact is made. If there is
enough current, skeletal muscle cells will be stimulated as rapidly as they can respond. This rate is
slower than 60 times per second. This will give a tetanic muscle contraction, resulting in the loss of
voluntary control of muscle movements. Cardiac muscle cells will receive 60 stimulations per second.
If the amplitude of the current is sufficient, ventricular fibrillation will occur. The heart is most
sensitive to such stimulation during the “vulnerable period” of the cardiac cycle that occurs during
much of the T wave.
In contrast, with direct current, there is a feeling of shock only when the circuit is made or broken
unless the voltage is relatively high.<snip>

[ejeffrey]

Pins inseteed just below the skin surface can get substantial current flow with just 10..30V

Actually not really.  There is an urban legend about a sailor being killed by a Simpson DMM puncturing the skin with the probes but this is easily refuted.

Get a small jar of water and prepare an isotonic (0.9% by weight) saline solution. Warm the water in a microwave to about 38C and insert your multimeter probes a few cm apart.  You will get a surprisingly large resistance because the spreading resistance in the neighborhood of the probes is still quite large.

Now get some copper braid and clip that to the probe ends and you will see much lower resistance due to the dramatically greater surface area. .

Make sure to thoroughly rinse your probes immediately afterwards to avoid corrosion.

[Gyro]

Okay, I’m old enough not to be embarrassed by this question. Let’s not get off topic and talk about AC and its issues, I’m sticking with DC for this question.

Unfortunately you cannot separate the two.

As Zero999 and jpanhalt say, it's AC that causes the heart to fibrilate and results in death at relatively low currents, certainly sub 100mA, depending on conditions - hand to hand vs same hand etc. At the same time, there are occasional stories on kids, and occasionally adults, straying onto 3rd rail railway lines and coming into contact with 600 -750V DC traction current on the conductor rail. These inevitably lead to severe burns. A lot of victims die from such injuries, particularly if they fall across the rail, but there are survivors, maimed and scarred but alive. From the severity of these burns, and calculating the likely current, far more than 100mA has passed.

At some point, you have to clarify what you mean by "If 100 milliamps can kill...". AC currents levels causing the heart to fibrilate, leading to rapid death are rather well documented. When it comes to DC current, injury due to severe burns, is highly variable both in survival chances and time to death. The survivability of burns [Edit: of any cause], fluid loss, infection etc. is far more variable, anecdotal stories become inevitable.

[Siwastaja]

Now get some copper braid and clip that to the probe ends and you will see much lower resistance due to the dramatically greater surface area. .

Exactly this. Sharp tips get better contact to the tissue by bypassing the skin, but at the same time, exactly due to the sharpness, you sacrifice all the surface area.

Safety extra low voltage limits are well thought out and any skin damage won't make them dangerous.